The invention relates to a high frequency heating apparatus with an electric heating device for the improvement of a method of heating and a construction for heating food material effectively and uniformly.
Generally, a high frequency heating apparatus with an electric heating device adopts two methods for heating food material, i.e., one method is to put food material in a heating chamber and heat it inwardly by heat energy generated from an electric heating device provided near the ceiling and the bottom of the chamber, and the other method is to directly irradiate food material in the heating chamber by high frequency waves supplied into the heating chamber so that food material generates heat outwardly. A selection as to which method to use is made depending on the type of food material and the cooking menu. Constructions of such conventional heating apparatuses are shown in FIGS. 1 to 5.
FIG. 1 is a cross-sectional view of a conventional high frequency heating apparatus with an electric heater, comprising a main body 1 provided with a heating chamber 2 which accommodates food material. Provided with heating chamber 2 are a food material supporting plate 3, a turn table 4 for preventing non-uniform heating of food material, an oven heater 5a and an oven and grill heater 5b, respectively disposed at the bottom and the ceiling of heating chamber 2 for heating food material by raising the temperature of heating chamber 2 and for performing oven and grill heating by means of radiant energy. Also provided in heating chamber 2 are high frequency oscillation devices, such as a magnetron 6 and a high voltage transformer 7. High frequency microwave energy is introduced into the heating chamber 2 through a supply opening portion 9 for supplying high frequency microwave energy provided at a portion in heating chamber 2, and conducted thereto by waveguide 8. The methods for heating food material by means of a high frequency heating apparatus with a heating device include a method using only high frequency microwaves, a method using high frequency microwaves and heaters 5a and 5b alternately, and a method using only heaters 5a and 5b. Thus, a preferable heating condition is obtained by selecting a heating method suitable for the particular food material and a cooking menu. When the food material is heated only by heaters 5a and 5b, as shown in FIG. 2, food material placed on a oven plate 10 can also be heated with the oven plate 10 made of metal and held approximately at the middle portion of heating chamber 2 by rails 11 mounted the sidewalls of heating chamber 2. Several rails 11 for holding the oven plate are longitudinally positioned stepwise along the sidewalls of heating chamber 2 to make it possible to shift the position of oven plate 10 so that food material can be heated most preferably depending on its size and the heating method employed. The output from heaters 5a and 5b can be adjusted to obtain a preferable heating condition in this heating apparatus.
In the case of oven and grill heating using heaters 5a and 5b, heating is performed after the atmosphere temperature in heating chamber 2 has been raised. Therefore, preheating must be performed in heating chamber 2, and the bulk temperature of food material is raised by high frequency microwaves with the material placed on the plate 3 and, thereafter, the material is placed on the oven plate 10 to heat it by heaters 5a and 5b in order to obtain a preferable condition by raising the temperature of heating chamber 2 and of the material. Thus, the heating operation according to this method, is very troublesome. Further more, a long time period is required to heat the material using only heaters, thus wasting energy. As is apparent, according to this method, the essential characteristics of a high frequency heating microwave apparatus cannot be attained, i.e., the heating operation is not easy, and high speed heating and energy saving cannot be accomplished. Heating of food material can be performed with the material placed on the plate 3 after heating by high frequency microwaves, but the advantage obtained by this method only saves the labor of shifting the material from the plate 3 to the oven plate 10. Further more, the time required for heating the material by this method is not so different from the time required when material heating is performed using only heaters 5a and 5b, and the amount of the material which can be cooked by one heating in this method is about 50% less than the amount possible when heating is performed using only heaters. For example, heating gratin for four men by this method must be performed twice. More specifically, the heating chamber 2 is usually rectangular in plan view, and the oven plate 10 is almost as large as heating chamber 2 so that heating chamber 2 is effectively used. However, the plate 3 is round in plan view and must be rotated to prevent high frequency microwaves from non-uniformly heating the material, and, thus, the quantity of material which absorbs heat considerably decrease. As shown in FIG. 3, it is supposed that easy operation, quick heating, and energy-savings can be accomplished when heating is performed by using a method of irradiating high frequency microwaves on the material with the oven plate 10 placed in the heating chamber 2 and thereafter, heating the material by heaters 5a and 5b. But, in practice, the heating chamber 2 is partitioned by the oven plate 10, so that the heating chamber 2 is small compared with the quantity of high frequency microwaves produced by the magnetron, and the electric field becomes very turbulent, which could cause, during high frequency microwave material heating, the material to be non-uniformly heated, abnormal heating in the heater 5b and insulator 12, sparks in the heater 5b because the electric field converges on projections such as the heater 5b and insulator 12 which holds heater 5b, during high frequency microwave heating, and increased electric wave leakage generated by the heater 5b in heating chamber 2. Thus, heating according to this method is very dangerous. Food material heating by the heaters is also disadvantageous because the material is non-uniformly heated when the heater 5b is positioned too close to the material, thus preventing fast heating. Therefore, safe, easy, fast, and energy efficient heating is very difficult to accomplish. This is particularly true for a grill heater.
In grill heater using radiant heat generated by heater 5b, there are two methods of heating food material depending on the material; on method is to heat the material by means of only heater 5b provided close to a ceiling, and the other one is where heater 5b is mainly auxiliary driven by heater 5a to raise the bulk temperature of the material so that a preferable heating condition can be obtained. But, in this method, of course, the material is mainly heated by heater 5b. In this method, when the quantity of food material and generated heat are constant, heating time periods and energy quantity required for heating the material are determined by the distance between the heater 5b and food material, the cubic measurement of the heating chamber partitioned by material placing plate 10 held by the sidewalls of heating chamber, the difference of radiant energy when a certain quantity of heat is generated by heater 5b, and a means for preventing heat energy from escaping from heating chamber 2. The time period required for heating is also important to obtain a preferable heating condition especially when grill heating is performed, so that the smaller a heating space is, and the higher heat temperature generated by heater 5b is, and the larger temperature retaining efficiency of the heating space is, the more effectively and uniformly food material is heated. Therefore, the position of rails 11 for holding material supporting plate 10, the insulation material 13 provided at the outside of heating chamber ceiling, and the to which the space is raised by heater 5b are commonly designed to obtain a speedy, energy efficient, and preferable heating condition.
A conventional heater 5b generally employs a sheathed heater whose configuration is shown in FIG. 4. As seen from the drawing, this configuration aims mainly at preventing various food materials from non-uniformly heating during oven heating. When heating is performed by placing food material adjacently to heater 5b to carry out fast, energy efficient heating, the material is scorched, as shown with scorch shading 14, in a short period of time at the portion close to the sheathed heater, and the material is not heated enough at the portion distant from the sheathed heater. Accordingly, when a sheathed heater 5b is placed close to the material, the material is very non-uniformly heated. When the material is placed distant from heater 5b to reduce the possibility of non-uniform heating of the material, radiant heat energy quantity per unit area is reduced, and the heating space is increased to a great extent. Consequently, fast, energy efficient heating cannot be accomplished, and in addition, heating is performed at low temperatures and the time period necessary for the heating is long. This is disadvantageous because grill heating must be performed after quickly raising the temperature of food material, resulting in that the surface of the material (food) becomes dry.
When food material is heated by high frequency microwaves, as shown in FIG. 1, the heating is performed with turn table 4, on which the material is placed and rotating to reduce the possibility of non-uniform heating of the material.
Likewise, when food material is heated by an electric heater, as shown in FIG. 2, heating is performed with the material placed on the oven plate 10 which is placed on rail 11 mounted on the sidewalls of heating chamber 2. At this time, heating is performed by two heaters 5a and 5b mounted on the bottom and the ceiling of heating chamber 2 to heat the material from the top and bottom thereof. Heaters 5a and 5b mounted on the bottom of heating chamber 2, are round, thereby causing the problem that heat generated by the heater 5a is collected under the turn table 4 and leads to the reduction of heat efficiency.
But this method has a disadvantage because the degree of heating of the center bottom portion of the material is much smaller than the other portions, causing non-uniform heating because convection currents and radiant heat generated by heater 5a flow only around the periphery of turn table 4.